REAL ESTATE HELD FOR INVESTMENT - Assets Concentration Risk (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($) ft² $ / ft²
Concentration Risk [Line Items]
Total Real Estate, Net	$ 141,477
Commonwealth Building | Assets, Total
Concentration Risk [Line Items]
Rentable Square Feet | ft²	224,122
Total Real Estate, Net	$ 66,006
Percentage of Total Assets	43.60%
Annualized Base Rent	$ 5,585
Average Annualized Base Rent per sq. ft. | $ / ft²	28.60
Occupancy	87.10%
The Offices at Greenhouse | Assets, Total
Concentration Risk [Line Items]
Rentable Square Feet | ft²	203,284
Total Real Estate, Net	$ 38,511
Percentage of Total Assets	25.40%
Annualized Base Rent	$ 4,206
Average Annualized Base Rent per sq. ft. | $ / ft²	20.69
Occupancy	100.00%
Institute Property | Assets, Total
Concentration Risk [Line Items]
Rentable Square Feet | ft²	155,385
Total Real Estate, Net	$ 36,960
Percentage of Total Assets	24.40%
Annualized Base Rent	$ 3,470
Average Annualized Base Rent per sq. ft. | $ / ft²	27.38
Occupancy	81.60%